QUANTITATIVE ESTIMATION FIG. 8. refilled after every two combustions, it is advisable to keep a little stock of solution in a bottle fitted with an ordinary cork. 6. A Calcium Chloride U- Tube.—The form of calcium chloride tube is shown in Fig. 8. It is fitted with sieved calcium chloride to within 2^ cm. (r in.) of the side pieces, and then with coarser pieces to within ^ cm. (£ in.). Place a small plug of cotton wool in both limbs above the chloride to keep it in position. Two well-fitting corks, cut off level with the glass and coated with sealing-wax, pro- duce an effective air-tight stopper to the open limbs, but it is preferable to seal them in the blow-pipe flame. The sealing requires a little skill. Carefully wipe off any chloride dust which may have adhered to the open ends of the two limbs. Cork up one limb and stopper one of the side tubes. Attach a short piece of rubber tubing to the other side tube to serve as a mouthpiece. Now soften the end of the open limb in a small blow-pipe flame, and at the same time heat the end of a short piece of glass rod. With the hot end of the rod gather up the edges of the open limb, and whilst rotating the limb backwards and forwards in the flame, draw it out and seal it up. If successful, the appear- ance of the tube is that shown in Fig. 9. The blob of glass is heated in a small flame, and, by gently blowing and re-heating and blowing again, the blob can be removed, and, finally, by using a rather larger flame, heating and blowing alternately, the end is neatly rounded. 7. A Porcelain or, preferably^ a Platinum Boat.—See that it slips easily into the combustion tube. The boat is kept in a desiccator on a flat cork or support made of glass rod when not in use. Preparation of the Tube.—Before starting the com- bustion it is necessary to clean and dry the combustion tube. This is effected by heating the whole length of the tube con- taining the copper oxide and spiral gradually to a dull red heat, and passing through it a slow stream of dry oxygen from the FIG. 9.